How a meteor shower forms
On certain nights each year, meteors arrive in much greater numbers than usual and all seem to fly out from the same part of the sky. These meteor showers happen when Earth passes through a stream of dust left behind by a comet, or occasionally by an unusual asteroid. Here is how such a stream forms and why it looks the way it does.
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Close to the Sun, a comet's ices turn to gas and carry dust away from the nucleus. Step 1 of 7A comet warms up near the Sun
A comet's nucleus is a lump of ice and dust a few kilometres across. Far from the Sun it is frozen and inactive. As its orbit brings it closer, within roughly the distance of the asteroid belt, sunlight warms the surface and ices such as water and carbon dioxide turn straight into gas.
The escaping gas drags dust grains out with it, from specks finer than flour to grains a few millimetres across and occasional larger crumbs. Together they make the glowing coma and the tail. These grains are the future meteoroids of a meteor shower.
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The dust drifts away from the comet slowly, so each grain follows almost the comet's own orbit. Step 2 of 7Grains leave on slightly different orbits
The dust leaves the nucleus slowly, at speeds of metres to perhaps a hundred metres per second, while the comet itself is racing around the Sun at tens of kilometres per second. So each grain ends up on an orbit almost the same as the comet's, but not quite.
Some grains are on orbits slightly larger than the comet's, which take a little longer to go round; others on slightly smaller ones, which take a little less time. Sunlight also pushes gently on the smallest grains, shifting their orbits further.
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Over a few orbits, the dust from one passage spreads ahead of and behind the comet into a narrow trail. Step 3 of 7Each passage lays a trail
Grains on slightly longer orbits gradually fall behind the comet, and those on slightly shorter ones pull ahead. After a few trips around the Sun, the dust from a single passage is drawn out into a long, narrow ribbon along the comet's orbit, called a dust trail.
Every time the comet returns, it lays a new trail. Young trails are narrow and dense. Older ones are longer and more spread out, and each has been nudged slightly differently by the planets, so the trails from different years do not lie exactly on top of one another.
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Over centuries, trails spread around the whole orbit and broaden into a meteoroid stream. Step 4 of 7Trails merge into a stream
Over hundreds and thousands of years, the trails stretch all the way around the orbit, and the gravity of the planets and the pressure of sunlight widen them. The result is a meteoroid stream: a tube of dust following the comet's orbit, sometimes millions of kilometres wide, with denser young trails embedded in it.
Streams outlast their comets. The Geminids come from 3200 Phaethon, which looks like an asteroid today, and the Quadrantids are linked to the asteroid 2003 EH1, probably a remnant of a comet that broke up. A stream can keep producing a shower long after its parent has faded or crumbled.
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Where Earth's orbit cuts through a stream, we meet it on the same dates every year. Step 5 of 7Earth crosses the stream
If a stream's orbit passes close to Earth's, Earth crosses it at the same point in its orbit, and so on the same dates, every year. Each grain it meets enters the atmosphere and burns up as a meteor. The Perseids, from Comet Swift–Tuttle, peak around 12 to 13 August; the Leonids, from Comet Tempel–Tuttle, around 17 November.
All the grains in a stream move in parallel at the same speed relative to Earth, so each shower has its own typical speed: about 59 kilometres per second for the Perseids, 71 for the Leonids, which meet Earth almost head-on, and about 35 for the slower Geminids. Shower meteoroids are small and fragile, so they burn up completely: no meteorite has ever been linked to a meteor shower.
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Parallel paths seem to spread out from one point, just as parallel railway lines seem to meet in the distance. Step 6 of 7Why showers have a radiant
The meteors of a shower all travel through the atmosphere on parallel paths. Seen from the ground, perspective makes parallel lines appear to spread out from a single distant point, just as railway lines seem to meet at the horizon. That point in the sky is the shower's radiant.
Showers are named after the constellation that holds their radiant: the Perseids from Perseus, the Geminids from Gemini, the Leonids from Leo. A meteor that does not trace back to the radiant is a sporadic meteor, unrelated to the shower. The radiant need not be where you look: shower meteors can appear anywhere in the sky.
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When Earth passes through a young, dense trail, rates can jump from tens to thousands per hour. Step 7 of 7Outbursts and storms
Most years a shower's rate is steady, because Earth passes through the broad, old stream. But sometimes Earth runs straight into a young, dense trail laid by the comet a few orbits earlier. Then rates can jump from tens per hour to hundreds or thousands: an outburst, or at its most extreme a meteor storm.
The Leonids produced famous storms in 1833 and 1966, with thousands of meteors per hour for a short time. Since the late 1990s, astronomers have been able to calculate where individual trails lie and predict such outbursts, sometimes to within minutes, as they did for the Leonid storms of 1999 to 2002.
Near the Sun a comet sheds dust on orbits slightly different from its own. Each passage leaves a narrow trail; over centuries the trails merge into a stream around the whole orbit. Where Earth crosses the stream we get a shower on the same dates each year, radiating by perspective from one point, with outbursts when we hit a young trail.
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